Chandra observations of radio-loud quasars at z > 4: X-rays from the radio beacons of the early universe

We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radio-quiet quasar (RQQ) at z ≈ 4.1-4.4. These observations cover a representative sample of RLQs with moderate radio loudness (R ≈ 40-400), filling the X-ray observational gap between optically selected RQQs (predominantly R ≲ 2-10) and the five known blazars at z > 4(R ≈ 800-27,000), where R = f _5GHz/f _4400Å (rest frame). We study the relationship between X-ray luminosity and radio loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic N _H, with a best-fit neutral intrinsic column density of N _H = 2.4 _-1.8 ^+2.0 × 10 ²² cm ^-2, consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low to moderate redshifts. Overall, the z ≈ 4-5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well.